The contrast ratio of images projected on front and rear projection screens is severely reduced by the ambient light present in the viewing environment. That is, the darkest level on the screen perceived by a user is affected by how much ambient light is directed to the user from the screen. The brightest level on the screen is determined by the power of the projector. The contrast ratio is the division of the brightest level by the darkest level. For example, in a movie theatre, when the room lights are on, the screen appears white or silver and this is the darkest image level available to the viewer. This effect is why the pre-show ads appear washed out. However, before the movie starts, the lights are accordingly dimmed or turned off and the screen appears dark, thus lowering this darkest level. This dimming of the ambient light is done in order to allow for the majestic beauty of the cinema presentation. However, in some environments, such as in conference rooms, churches, and seminars, there is a need to keep the ambient light on to allow for note taking, participant movement, or to maintain conversational awareness.
Prior approaches to reducing the effects of ambient light have used grey screens to improve the contrast level, but this technique also reduces the overall brightness of the image. Accordingly these grey screens required a more expensive projector that could cast more light to compensate. Another prior technique involved modification of surface geometry of the screen to include a transparent diffusion layer in front of a reflective layer. This approach had the effect of focusing more of the reflected projector light into a limited viewing cone, which is called screen gain. Outside of this viewing cone, the picture quality dropped while inside the viewing cone, the brightness increased with limited effect on improving the contrast ratio as the ambient light also was affected by the screen gain. Some high-gain projection screens utilized an array of lenses over a reflective background to direct projected light back to a viewer. These screens did preferentially reject ambient light with respect to projected light but suffered from a severely limited viewing angle and tended to be relatively expensive.
Regretfully, most people choose to just live with the reduced contrast ratio rather than pay the exponentially increasing cost of more powerful projectors and custom screens. If only there were a better way, audience acceptance of projected images could be improved to better compete with direct view displays.